[erlang-questions] link/1 appears to interfere with message sending

Garret Smith <>
Tue Mar 19 19:10:09 CET 2013


The Javadoc of OtpMsg.type() is what you're looking for.

The Java node is receiving a notification of the process link.  This
message type has no "payload", so getMsg() fails with NPE.  Before calling
getMsg(), check the message type with msg.type().

-Garret Smith


On Fri, Mar 15, 2013 at 11:15 PM, J David Eisenberg <
> wrote:

> I have written a test program that connects Erlang and Java as follows:
> 1) Erlang module spawns a Java program via open_port/2.
> 2) Erlang then uses the port to send Java the Erlang node name.
> 3) The Java program uses that information to open a connection to Erlang.
> 4) Java sends back the pid of the Java program, then awaits a message
> from Erlang
> 5) Erlang sends back an acknowledgment to Java, and...
> 6) Java sends back a final message to Erlang.
>
> Here is the Java code with debugging output in Test.java
> //==========================
> import com.ericsson.otp.erlang.*;
> import java.io.BufferedInputStream;
> import java.io.BufferedReader;
> import java.io.InputStreamReader;
>
> public class Test
> {
>   public static void main(String[] args)
>   {
>     OtpSelf self;
>     OtpConnection connection;
>     OtpPeer erlangPeer;
>     OtpErlangObject [] elements = new OtpErlangObject[2];
>     OtpMsg message;
>     OtpErlangObject obj;
>     OtpErlangTuple tuple;
>     OtpErlangPid erlangPid;
>     String erlangAtom;
>
>     try
>     {
>       self = new OtpSelf("java");
>       BufferedReader input = new BufferedReader(new InputStreamReader(
>         System.in));
>       String erlNodeName = input.readLine();
>       System.err.println("Erlang node is |" + erlNodeName + "|\r");
>
>       erlangPeer = new OtpPeer(erlNodeName);
>
>       connection = self.connect(erlangPeer);
>       System.err.println("Connection to " + erlangPeer + " " +
> connection + "\r");
>
>       elements[0] = new OtpErlangAtom("sent_from_java");
>       elements[1] = connection.self().pid();
>       connection.send("testing", new OtpErlangTuple(elements));
>
>       message = connection.receiveMsg();
>       System.err.println("Java received message " + message + "\r");
>       obj = message.getMsg();
>       tuple = (OtpErlangTuple) obj;
>       erlangAtom = ((OtpErlangAtom) tuple.elementAt(0)).atomValue();
>       erlangPid = (OtpErlangPid) tuple.elementAt(1);
>       System.err.println("Java received " + erlangAtom + " " +
> erlangPid + "\r");
>
>       connection.send("testing", new OtpErlangAtom("goodbye"));
>     }
>     catch (Exception e)
>     {
>       System.err.println("Java gets exception" + "\r");
>       e.printStackTrace(System.err);
>     }
>   }
> }
> //===========================
>
> And here is the Erlang module, test.erl:
> %=====================
> -module(test).
> -export([go/0,test/0]).
>
> go() ->
>   Pid = spawn(?MODULE, test, []),
>   register(testing, Pid),
>   io:format("Erlang spawns test process with pid ~p~n", [Pid]),
>   process_flag(trap_exit, true),
>   Port = open_port({spawn, "java -cp
> .:/usr/lib/erlang/lib/jinterface-1.5.6/priv/OtpErlang.jar:core.jar
> Test"},
>     [{line, 256}]),
>
>   % send my node name to the Java program.
>   port_command(Port, list_to_binary(atom_to_list(node()) ++ "\n" )),
>   test().
>
> test() ->
>   receive
>     % get the pid of the Java process
>     {Atom, Pid} ->
>       io:format("Erlang receives: ~p ~p~n", [Atom, Pid]),
>
>       %%% link(Pid), % let me know if Java crashes..
>
>       Pid ! {sent_from_erlang, self()}, % and send it a message
>       test(); % then wait for another message
>     Other ->
>       io:format("Erlang receives ~p~n", [Other])
>    end.
> %======================
>
> This all works great until I uncomment the ilne with the %%%
> link(Pid). If I do that, then, when I send the message to Java, it
> says:
>
> java.lang.NullPointerException
> at com.ericsson.otp.erlang.OtpMsg.getMsg(OtpMsg.java:212)
> at Test.main(Test.java:39)
>
> If I link to the Port instead of the Pid, everything works. I'm
> clearly overlooking some simple concept; what is it?
> _______________________________________________
> erlang-questions mailing list
> 
> http://erlang.org/mailman/listinfo/erlang-questions
>
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